[tox] Update doc testenv to account for documentation restructuring * Build doc from `doc` directory. * Use `sphinx-build` directly instead of `make` as this now works and allows to build within .tox temporary environment (instead of doc/_build), preferable for a "test" action. Related to #5447161.

* {
  color: #000 !important;
}

div#popupLoginBox,
div#popupLoginBox,
img#logo, div.header,
#navColumnLeft, #navColumnRight,
#footer {
  display: none
}

div#pageContent{
  border: none;
}